Multiple Input Multiple Output (MIMO) technique is a technique that increases the performance in a wireless communication system and is an integral part of the 3rd and 4th generation wireless systems. 5G wireless systems will also employ very large MIMO systems (also known as massive MIMO systems) comprising hundreds of antennas.
In closed loop spatial multiplexing (SM) schemes a user equipment (UE) provides feedback on a channel between the UE and a node of the wireless system, e.g. an evolved NodeB (eNB) in case of a wireless system implementing Long Term Evolution, LTE, standard. Examples of such feedback in the LTE system comprise Channel Quality Indicator (CQI), Precoding Matrix Indicator (PMI) and rank indicator (RI). CQI indicates the current status of the channel quality, PMI indicates a preferred precoding matrix of a predefined codebook for use by the eNB and RI defines how many spatial layers, i.e. number of independent information streams, that the UE is able to decode.
The performance of the closed loop MIMO system generally improves with the cardinality (size) of the codebook. In general, finding the RI and PMI is cumbersome and involves many computations. For example in 4 Tx MIMO (4 transmitting antennas), the UE needs to search 64 precoding entities of the codebook in order to find the RI and PMI. Such search entails several drawbacks. The search is time consuming, increases power consumption and hence drains UE battery life. For the UE to be able to perform the search, it has to have a fast and powerful processing unit and memory capacity which increases the cost thereof.